recovery: Allow separating device asserts with a colon too

* android::base::Split already supports multiple separators

* Make FINGERPRING_SEPARATOR a #define so that we can easily
  append the other separator. Why even was it ever a char* ?

  Note that we cannot directly make FINGERPRINT_SEPARATOR use ':'
  as a separator because, as the name suggests, it's also used to
  split build fingerprints which include ':' as a valid character.

Change-Id: I101e394d8e9fd7f2cbfcf01314e9bda38146c7d5